1. Field of the Invention
This invention relates to an engine fuel control apparatus equipped with an exhaust purification catalyst which controls an average air/fuel ratio prior to an ignition in an in-cylinder combustion chamber to be leaner than a stoichiometric air/fuel ratio in a predetermined region of low rotational speed and low load side and performs a fuel cut control under a predetermined condition.
2. Description of Related Arts
Conventionally, this kind of engine fuel control apparatus is known and is provided with a fuel injector so as to face an in-cylinder combustion chamber of the engine, causes the fuel injector to inject fuel mainly during a cylinder compression stroke to generate a stratified combustion state when the engine is in a predetermined region of low rotational speed and low load side and causes the fuel injector to inject fuel mainly during a cylinder intake stroke in a region of high rotational speed or high load side, as disclosed in Japanese publication of patent application no. 11-229856.
In the aforementioned stratified combustion state, since a mixture combusts in a state where the mixture is unevenly distributed in a proximity of a spark plug, an average air fuel ratio prior to an ignition in an in-cylinder combustion chamber, hereinafter referred to as the in-cylinder air/fuel ratio, can be set at an extremely lean state such as A/F=30-140, thereby fuel consumption rate can be substantially improved by reducing pumping loss or heat loss compared with a homogeneous combustion state.
Then, when the in-cylinder air/fuel ratio is in the extremely lean state as described above, an exhaust air/fuel state is in a lean state as well. In other words, generally, if the in-cylinder air/fuel ratio is the substantially stoichiometric air/fuel ratio, oxygen concentration remaining in the exhaust gas will become less than about 0.5% and an exhaust air/fuel state at this time will become a state corresponding to the substantially stoichiometric air/fuel ratio. On the other hand, if the in-cylinder air/fuel ratio becomes lean, the exhaust air/fuel state becomes lean, i.e. a higher oxygen concentration state. But, it is very difficult to efficiently reduce NOx in this higher oxygen concentration state.
Hence, in the aforementioned prior art, NOx catalyst is provided including NOx absorbent which absorbs NOx in an excess oxygen atmosphere and releases NOx when the oxygen concentration decreases, and in the proximity of this a three-way catalyst is disposed, wherein the aforementioned NOx catalyst is caused to absorb NOx when the exhaust air/fuel state is lean while the three-way catalyst is caused to reduce and purify NOx released from the NOx catalyst when the exhaust air/fuel state becomes the stoichiometric air/fuel ratio or richer.
In this respect, generally, such a NOx absorbent as described above has a characteristic of lower absorptive ability for NOx the greater the NOx absorptive amount. Hence, in the aforementioned prior art, when an engine operation in the stratified combustion state continues over a long period of time, it is shifted to the homogeneous combustion state and the exhaust air/fuel state is made richer by compulsorily changing the in-cylinder air/fuel ratio into the rich side prior to the substantial decrease of the NOx absorptive performance by the NOx catalyst, to release NOx from the NOx catalyst and reduce and purify it with the three-way catalyst (hereinafter referred to as a compulsory NOx purge).
Also in the aforementioned prior art, when the engine is in a decelerating operating state and when a predetermined condition is fulfilled such as no manipulation of an accelerator pedal being made, a fuel cut control is performed which stops a fuel injection supply from a fuel injector, whereby an engine brake effect can be enhanced and a fuel consumption rate can be decreased.
Meanwhile, since an exhaust temperature state generally becomes lower compared to the time of the substantially stoichiometric air/fuel ratio as the in-cylinder air/fuel ratio becomes leaner, a temperature state of catalyst tends to be lower in a spark ignited direct injection engine which is in the stratified combustion state in the lower rotational speed and lower load region as in the aforementioned prior art and a diesel engine whose in-cylinder air fuel ratio becomes lean during almost all of the operating region. Furthermore, when the fuel cut control is performed as in the aforementioned prior art, thereby the aforementioned catalyst temperature state becomes extremely low and the exhaust purification performance by this catalyst might be deteriorated.
Also if the compulsory NOx purge were performed regardless of the engine operating condition as in the aforementioned prior art, when the engine is in an operating condition where vibration or noise is inherently low such as an idle operating condition, the combustion state might be compulsorily shifted from the stratified combustion through the homogeneous combustion to the stratified combustion and an output torque might vary. Even if the torque variation at this time is small, since it occurs regardless of driver manipulation, a driver tends to get a strange feeling and a driving feeling would be deteriorated if the occurring frequency were high.
It is an object of the present invention to improve the driving feeling in the overall engine operating region while maintaining the exhaust purification performance of for example a catalyst by elaborating a control process of the exhaust air/fuel state in terminating the fuel cut control and recovering the fuel supply in an engine fuel control apparatus which operates the engine in the lean in-cylinder air/fuel state in the lower rotational speed and lower load region and performs the fuel cut control under a predetermined condition.
Preferably, a recovery timing is determined when the engine shifts from a fuel cut state to an operating condition of lean in-cylinder air/fuel ratio and then controls an exhaust air/fuel state to be a state corresponding to or richer than a substantially stoichiometric air/fuel ratio if a catalyst is in a state of lower purification performance due to being over cooled or an increase of NOx absorptive amount.
Particularly, according to an aspect of the present invention, there is provided a fuel control apparatus of an engine equipped with a catalyst which performs a three-way purification function at least when an exhaust air/fuel state is a state corresponding to a substantially stoichiometric air/fuel ratio, an in-cylinder air/fuel ratio control device for controlling an in-cylinder air/fuel ratio which is an average air fuel ratio prior to an ignition in an in-cylinder combustion chamber to be leaner than the stoichiometric air/fuel ratio at least when the engine is in a predetermined region of low rotational speed and low load side, and a fuel cut control device for performing a fuel cut control for compulsorily stopping a fuel supply to the combustion chamber under a predetermined condition during a decelerating operation of the engine. And it is configured to comprise a low catalyst temperature state determining device for determining if the catalyst is in a predetermined low temperature state where its exhaust purification performance is deteriorated; and an exhaust air/fuel ratio control device for controlling the exhaust air/fuel state to be a state corresponding to or richer than the substantially stoichiometric air/fuel ratio when the low catalyst temperature determining device determines that the catalyst is in the low temperature state and when the fuel cut control by the fuel cut control device terminates and then the engine shifts to the predetermined region. At that time, the air/fuel ratio is preferably controlled between the stoichiometric air/fuel ratio and an air fuel ratio of 12 because unburned fuel in the exhaust gas increases the temperature of the catalyst if the air/fuel ratio exceeds 12.
Accordingly, when the engine is in the predetermined region of low rotational speed and low load side, the in-cylinder air/fuel ratio of the engine is controlled to be leaner than the stoichiometric air/fuel ratio so as to reduce the fuel consumption rate. Also when the predetermined condition is established in the decelerating operation of the engine, the fuel cut control device performs the fuel cut control to prevent the wasteful fuel consumption. During this fuel cut control, air induced into the combustion chamber of engine will be discharged into the exhaust passage whereby the temperature state of the catalyst will rapidly decrease.
When the temperature state of the catalyst becomes lower as described above and becomes the predetermined lower temperature state where the exhaust purification performance is deteriorated, this state is determined by the low catalyst temperature state determining device. And when the fuel cut control terminates and the engine shifts into the predetermined region, the exhaust air/fuel state is controlled to be the condition corresponding to or richer than the substantially stoichiometric air/fuel ratio (hereinafter also referred to as enrichment).
That is, in the operating region where the exhaust air/fuel state becomes inherently lean, the exhaust air/fuel state is temporarily enriched in the recovery from the fuel cut control, the exhaust temperature will be increased, whereby the temperature state of the catalysts can be increased to prevent a deterioration of exhaust purification performance. Also, the driver would not have a strange feeling even if the output torque of the engine varies due to the enrichment of air/fuel ratio because of the inherent recovery timing where a fuel injection mode changes.
Next, according to another aspect of the present invention, there is provided a fuel control apparatus of an engine equipped with a NOx absorbent which absorbs NOx in exhaust gas of excess oxygen atmosphere and releases the absorbed NOx due to a decrease of oxygen concentration, a catalyst which performs a three-way purification function at least when an air/fuel state of exhaust gas is a state corresponding to a substantially stoichiometric air/fuel ratio, an in-cylinder air/fuel ratio control device for controlling an in-cylinder air/fuel ratio which is an average air fuel ratio prior to an ignition in an in-cylinder combustion chamber to be leaner than the stoichiometric air/fuel ratio at least when the engine is in a predetermined region of low rotational speed and low load side, and a fuel cut control device for performing a fuel cut control for compulsorily stopping a fuel supply to the combustion chamber under a predetermined condition during a decelerating operation of the engine. And it is configured to comprise a NOx absorptive state determining device for determining if NOx absorptive amount of the NOx absorbent is equal to or more than a predetermined amount, and an exhaust air/fuel ratio control device for controlling the exhaust air/fuel state to be a state corresponding to or richer for the substantially stoichiometric air/fuel ratio when the NOx absorptive state determining device determines that the NOx absorptive amount exceeds the predetermined amount and when the fuel cut control by the fuel cut control device terminates and then the engine shifts to the predetermined region.
Accordingly, the engine is operated in the leaner in-cylinder air/fuel state when in the predetermined region of low rotational speed and low load side. In the operating condition of leaner air/fuel ratio, NOx in the excess oxygen atmosphere is absorbed in the NOx absorbent and when the NOx absorptive amount of this NOx absorbent is equal to or more than the predetermined amount, this condition is determined by the NOx absorptive state determining device. And if this determination is made, the exhaust air/fuel ratio control device controls the exhaust air/fuel state to be richer when the fuel cut control terminates and the engine shifts into the predetermined region.
By this, the exhaust air/fuel state is temporarily enriched at the time of recovery from the fuel cut control in the operating region where the exhaust air/fuel state is inherently lean then NOx is released from the NOx absorbent and is reduced and purified by the catalyst. Also due to the timing of the fuel recovery, the driver does not often have the strange driving sensation. That is, NOx purging at the timing of fuel recovery can improve a driving sensation while maintaining the exhaust purification performance of the NOx absorbent or the catalyst by decreasing a frequency of compulsory NOx purging in the lean air/fuel ratio operation.
According to another aspect of the present invention, the exhaust air/fuel ratio control device correctively controls the in-cylinder air/fuel ratio to be the substantially stoichiometric air/fuel ratio or richer and it comprises a throttle valve for controlling intake air amount into the in-cylinder of the engine, a throttle opening control device for controlling according to an accelerator manipulative amount and an engine operating condition and closing the throttle valve during the execution of the fuel cut control by the fuel cut control device, and a throttle opening operation regulating device for regulating the control of the throttle valve opening operation by the throttle valve opening control device if the accelerator operating amount is zero when the corrective control of in-cylinder air fuel ratio by the exhaust air/fuel ratio control device is executed.
Accordingly, during the execution of the engine fuel cut control, closing of the throttle valve by the throttle valve opening control device decreases the exhaust air amount from the combustion chamber as much as possible and hereby cooling of the catalyst is suppressed. Also the corrective control is executed to cause the in-cylinder air/fuel ratio to be rich when the engine returns from the fuel cut control to the predetermined lean air/fuel ratio region. At this time, if the accelerator manipulative amount is zero, that is, the engine returns to an idle operating condition, the throttle valve opening regulation device regulates the opening operation control of the throttle valve and the throttle valve opening is maintained in a relatively small state. Hereby, a torque variation due to the corrective control of the in-cylinder air/fuel ratio to the rich side can be suppressed.
According to another aspect of the invention, a fuel injector is provided which injects fuel directly into the in-cylinder combustion chamber, the fuel cut control device terminates the fuel cut control when the engine rotational speed becomes less than a predetermined recovery rotational speed, and the exhaust air/fuel ratio correction device causes the fuel injector to additionally inject fuel during a cylinder expansion stroke or a cylinder exhaust stroke so that the exhaust air/fuel state becomes a state corresponding to or richer than the substantially stoichiometric air/fuel ratio. At that time, the air/fuel ratio is preferably controlled between stoichiometric air/fuel ratio and an air fuel ratio of 12 because unburned fuel in the exhaust gas decreases the temperature of the catalyst if the air/fuel ratio exceeds 12. Moreover, the present invention comprises a throttle valve for controlling an intake air amount of the engine, and a throttle opening control device for closing the throttle valve when the fuel cut control is performed by the fuel cut control device and causing the throttle valve to operate to open even if the fuel cut control is performed when the engine rotational speed decreases to a set rotational speed higher than the recovery rotational speed if the catalyst is determined to be in the low temperature state by the low catalyst temperature determining device.
Accordingly, during the execution of the engine fuel cut control, the closing the throttle valve by the throttle valve opening control device decreases the exhaust air flow from the combustion chamber as much as possible whereby cooling of the catalyst is suppressed. And when the engine rotational speed decreases to the predetermined rotational speed during the execution of this fuel cut control, the throttle opening control device operatively opens the throttle valve. Following this, when the engine rotational speed decreases to the recovery rotational speed, that is, the fuel cut control terminates and fuel injection supply to the combustion chamber by the fuel injector is resumed, an intake air amount to the combustion chamber will sufficiently increase.
At this time, the fuel injector makes the additional injection of fuel during the cylinder expansion stroke or exhaust stroke to make the exhaust air/fuel state. That is, a large amount of high temperature exhaust is supplied to the catalyst by sufficiently increasing the exhaust flow and enriching an air/fuel state of that exhaust at the time of the recovery from the fuel cut condition and an temperature state of that can be rapidly increased. Further, when the fuel is additionally injected during the cylinder expansion stroke or exhaust stroke, rotational increase (torque increase) caused by the injection is small and a rotational variation (torque variation) is small even at the timing of the termination of the additional injection.
According to another aspect of the invention, a fuel injector is provided which injects fuel directly into the in-cylinder combustion chamber, the fuel cut control device terminates the fuel cut control when the engine rotational speed becomes less than a predetermined recovery rotational speed, and the exhaust air/fuel ratio correction device causes the fuel injector to additionally inject fuel during a cylinder expansion stroke or a cylinder exhaust stroke so that the exhaust air/fuel state is the state corresponding to the substantially stoichiometric air/fuel ratio or richer. At that time, the air/fuel ratio is preferably controlled between stoichiometric air/fuel ratio and an air fuel ratio of 12 because unburned fuel in the exhaust gas decreases the temperature of the catalyst if the air/fuel ratio exceeds 12. Moreover, the present invention comprises: a throttle valve for controlling an intake air amount of the engine; a low catalyst temperature determining device for determining if the catalyst is in a predetermined low temperature state where its exhaust purification performance is deteriorated; and a throttle opening control device for closing the throttle valve when the fuel cut control is performed by the fuel cut control device and causing the throttle valve to operate to open even if the fuel cut control is performed when the engine rotational speed decreases to a set rotational speed higher than the recovery rotational speed if the catalyst is determined to be in the low temperature state by the low catalyst temperature determining device.
Accordingly, the same function and effect as the aforementioned aspect is obtained. That is, at the time of resuming the fuel injection supply to the combustion chamber, an intake air amount to the combustion chamber is sufficiently increased and then the exhaust air/fuel state can be enriched, thereby NOx purge utilizing the fuel recovery timing is sufficiently enhanced and a catalyst activity can be more improved.
Next, according to another aspect of the present invention, there is provided a fuel control apparatus of an engine equipped with a catalyst performing a three-way purification function at least when an air/fuel state of exhaust gas is in a state corresponding to a substantially stoichiometric air/fuel ratio; an in-cylinder air/fuel ratio control device for controlling an in-cylinder air/fuel ratio which is an average air fuel ratio prior to an ignition in an in-cylinder combustion chamber to be leaner than the stoichiometric air/fuel ratio at least when the engine is in a predetermined region of low rotational speed and low load side; and a fuel cut control device for performing a fuel cut control of compulsorily stopping a fuel supply to the combustion chamber under a predetermined condition during a decelerating operation of the engine. And it is configured to comprise a duration determining device for determining if a duration of the fuel cut control by the fuel cut control device is equal to or more than a set time; and an exhaust air fuel/ratio control device for controlling an exhaust air/fuel state to be a state corresponding to a substantially stoichiometric air/fuel ratio or rich of that when the fuel cut control terminates and then the engine shifts to the predetermined region if the duration is determined to be equal to or more than the set time. At that time, the air/fuel ratio is preferably controlled between stoichiometric air/fuel ratio and an air fuel ratio of 12 because unburned fuel in the exhaust gas increases the temperature of the catalyst if the air/fuel ratio exceeds 12.
Accordingly, when the engine is in the predetermined region of lower rotational speed and lower load side, the in-cylinder air fuel ratio is controlled to be leaner than the stoichiometric air/fuel ratio and also the fuel cut control executed under the predetermined condition will cause a catalyst temperature state to rapidly decrease. And when the duration of the fuel cut control is equal to or more than the set time and this is determined by the duration determining device, the control by the exhaust air/fuel ratio control device and the exhaust air/fuel ratio is enriched.
That is, the longer the duration of the fuel cut control, the longer the catalyst will be cooled, and consequently the engine rotational speed is relatively high and air flow of the catalyst is large when the fuel cut control is commenced. Accordingly, the catalyst temperature state is very low at the time of termination of the fuel cut control.
Therefore, in this case, enriching the exhaust air/fuel state at the timing of recovery from the fuel cut control can prevent the deterioration of the exhaust purification performance by increasing the catalyst temperature state while reducing strange driving sensation felt by the driver.